During the deposition of Pentacene on a Si-SiO2 gate structure with Au bottom contacts for source and drain, the film growth was monitored with simultaneous in situ macro Raman spectroscopy and drain current measurements of the OFET device. The deposition of the active layer was carried out under UHV conditions at a growth rate of 0.65 Å/min. The purpose of the in situ characterization was to determine the minimum nominal thickness of the Pentacene layer required for efficient charge transport through the OFET circuit. At a thickness around 1.5 nm nominal coverage, the first percolation paths through the first organic monolayer develop, resulting in a sharp rise of the drain current. Up to a nominal film thickness of 30 nm, a subsequent slower increase of the drain current can be observed, revealing that the percolation of the first monolayer continues on a slower pace up to rather thick organic layers. These in situ measurements were complemented by ex situ isothermal deep level transient spectroscopy (charge QTS).
Potentiometry with a Kelvin probe atomic force microscope is used to investigate the contact resistances of pentacene OFETs, so that the injection of the charges at the source contact and their extraction at the drain contact can be distinguished from the influence of trap
states on the charge transport through the accumulation channel. The
samples consist of Au bottom contacts on a SiO2 gate dielectric with a channel length of L=10- 15 μm and a channel
width of W=100 μm. The gate oxide is first treated by an
oxygen plasma before depositing about 30 nm of pentacene under high
vacuum conditions. The output characteristics are measured as a
function of temperature in an evacuated cryostat, revealing
temperature-activated hole transport. The potentiometry measurements
are performed ex situ under atmospheric conditions after storing
the samples in air for several weeks. At room temperature, the
pentacene OFETs are dominated by the resistance at the injection
contact, so that the mobility in the channel region as deduced from
potentiometry is about one order of magnitude higher than the value
obtained from the output characteristics. The measurements are
interpreted with microscopic model calculations for the
temperature-activated currents.
The understanding of metal/organic interfaces is fundamental for achieving high quality contacts in organic based field effect transistors,OFETs. Here the formation of In and Ag interfaces on 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA)and N,N dimethy l3,4,9,10 perylene tetracarboximide (DiMe-PTCDI)is investigated by in situ Raman spectroscopy. Upon metal deposition a significant enhancement of the Raman signals arising
from internal vibrational modes is observed for these two perylene derivatives,clearly indicating the presence of surface enhanced Raman scattering (SERS). Molecules in direct contact with the metal are involved in a weak dynamical charge transfer in the electronic ground state, resulting in a breakdown of the Raman selection rules for the free molecule. Both metals show very weak interactions with the organic thin films. This finding is opposite to previous suggestions of covalent bond formation between In and O atoms in PTCDA. The Raman
scattering of molecules close to the metal interface is enhanced by coupling to the plasmon resonance of small metal clusters,indicating a high roughness of the metallic film. The enhancement factors can be used to extract information on diffusion of the metal atoms into the organic film and on the morphology of the contact layer.
Raman spectroscopy is capable of distinguishing neutral and charged states of organic molecules due to its sensitivity for charge induced changes in the molecular geometry and the bond strength.Combined with in situ electrical measurements it provides a powerful tool for characterizing charged molecules in the channel of organic field effect transistors OFETs. The active 3nm C60 layer in an OFET structure with bottom gate con figuration was characterized by in situ Raman spectroscopy using the 514.5 nm (2.41 eV)Ar+ laser line as a function of the drain source (Vds) voltage. The Raman spectra show pronounced changes upon application of a drain source Vds voltage. The experimental findings are compared to the vibrational spectra calculated for molecules under external bias fields. Complementary to the Raman characterization,the Id-Vds and Id-Vgs characteristics were recorded and the mobility value of 0.1 cm2/V s was derived from these measurements.
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